Mutation analysis of 12 genes in Chinese families with congenital cataracts.

PURPOSE
To identify mutations in 12 genes in Chinese families with congenital cataracts.


METHODS
Twenty five families with congenital cataracts involved in this study. The coding exons and adjacent intronic regions of 12 genes were analyzed by cycle sequencing, including the alpha A crystallin (CRYAA), alpha B crystallin (CRYAB), beta A1 crystallin (CRYBA1), beta A4 crystallin (CRYBA4), beta B1 crystallin (CRYBB1), beta B2 crystallin (CRYBB2), beta B3 crystallin (CRYBB3), gamma C crystallin (CRYGC), gamma D crystallin (CRYGD), gamma S crystallin (CRYGS), alpha 3 gap junction protein (GJA3), and alpha 8 gap junction protein (GJA8) genes. Novel variants were further evaluated in 96 normal controls.


RESULTS
Nine mutations were identified in 10 of the 25 families (40%), including 5 novel (c.350_352delGCT in CRYAA, c.205C>T in CRYAB, c.106G>C in CRYGD, c.77A>G in CRYGS, c.1143_1165del23 in GJA3) and 4 known (c.292G>A in CRYAA; c.215+1G>A and c.272_274delGAG in CRYBA1, and c.176C>T in GJA3). All novel mutations were predicted to be pathogenic and were not present in 96 controls.


CONCLUSIONS
Mutations in the 12 genes encoding crystallins and connexins were responsible for 40% Chinese families with congenital cataracts. Our results enriched our knowledge on the molecular basis of congenital cataracts in Chinese population.

Congenital cataract is a leading cause of childhood blindness, accounting for 10~38% of blindness in children. The prevalence of congenital cataract is estimated to be 0.6~6 per 10,000 live births [1,2]. Various etiological factors have been reported, including infection, neonatal asphyxia, malnutrition, and genetic defects. It was reported that 8.3%~25% of congenital cataracts were inherited [3]. Several genes have been identified to be associated with congenital cataracts [3,4], such as genes encoding crystallins, connexins and other membrane proteins, beaded filament proteins, growth and transcription factors, and others.
In this study, molecular genetic analysis was performed on all these 12 genes in 25 Chinese families with congenital cataract. Nine mutations, including 5 novel and 4 known, were identified in 10 of the 25 families (40%).

Patients:
Written informed consent conforming to the tenets of the Declaration of Helsinki and following the Guidance of Sample Collection of Human Genetic Diseases (863-plan) by the Ministry of Public Health of China were obtained from the participating individuals or their guardians before the study. Twenty five Chinese families with congenital cataracts were enrolled in this study. Genomic DNA was prepared from leukocytes of peripheral venous blood. This study was approved by the Institutional Review Board of the Zhongshan Ophthalmic Center, Sun Yat-sen University, Guangzhou, China. Mutational screening: Bioinformation of the 12 genes was listed in Table 1, which was obtained from the National Center for Biotechnology Information (NCBI). Polymerase chain reaction (PCR) was used to amplify the coding exons and adjacent intronic sequences of the 12 genes. The primer sequences used to amplify each coding exon and its adjacent regions of the 12 genes were referred to the previous publications [36,[57][58][59] with modification for some primers (Appendix 1). The nucleotide sequences of PCR products were determined with the ABI BigDye Terminator cycle sequencing kit v3.1 on a genetic analyzer (ABI Applied Biosystems, Foster City, CA). Variations were identified by importing the sequencing results from patients and consensus sequences from the NCBI human genome database into the SeqManII program of the Lasergene package (DNAStar Inc., Madison, WI). Potential variants detected in patients were further evaluated in 96 normal controls by cycle sequencing.
Database and online tools: Mutation description followed the recommendation of the Human Genomic Variation Society (HGVS). The effects of novel missense mutations on the encoded proteins were further evaluated by Polymorphism Phenotyping (PolyPhen-2) [60,61] and Sorting Intolerant From Tolerant (SIFT) [62] at the protein level.

RESULTS
Based on complete analysis of the coding exons and their adjacent intronic regions in the 12 genes in 25 families, nine heterozygous mutations were detected in 6 genes in 10 families (Table 2, Figure 1 Three of the five novel variants were missense mutations. Of the three, the c.205C>T (p.Arg69Cys) in CRYAB and the c.77A>G (p.Asp26Gly) in CRYGS involved highly conserved residues while the other one (p.Ala36Pro in CRYGD) replaced a nonconserved residue (Figure 3). The novel c. 350_352delGCT mutation in CRYAA resulted in substitution of arginine at position 117 and deletion of tyrosine at position 118, where the two residues are highly conserved (Figure 3). The c.1143_1165del23 mutation resulted in frameshift with additional 48 new residues from residue 381.
The clinical data of the available patients with mutations were listed in Table 3 and cataract phenotypes for some patients were shown in Figure 4.

DISCUSSION
In this study, nine heterozygous mutations in the 12 genes encoding crystallins and connexins were identified in 10 out of 25 Chinese families (40%) with congenital cataract. Eight of the 25 families (32%) had mutations in crystallin genes and two of them (8%) had mutations in connexin genes.
For the congenital cataract families with identified mutations reported before, about three-fourths of the families had mutations in the 10 crystallin and 2 connexin genes, which was mostly based on studies of individual gene [3]. However, this mutation proportion may not reflect the actual frequency of these genes in congenital cataract, since the genes encoding crystallins and connexins might be more frequently analyzed. In fact, varied frequencies of mutation detection, mostly lower, were reported in several studies involving analysis of multiple genes. Devi et al. found crystallin mutations in 16.6% (10 of 60) Indian families by analyzing the 10 crystallin genes [10]. Burdon et al. [63] detected crystallin mutations in 5.3%  [36,50] *Reported indicates that these known mutations have been reported previously in other families.       (2/38) Australian families by analyzing 7 crystallin genes. Hansen et al. [36] detected crystallin and connexin mutations in 35.7% (10/28) and 21.4% (6/28) Danish families, respectively. Kumar et al. [40] detected mutations in 20% (6/30) Indian families by analyzing 4 of the 12 genes. Wang et al. [29] identified mutations in 15% (3/20) Chinese families by analyzing 10 of the 12 genes. None of these studies performed analysis of all the 12 genes in the same set of families. In this study, we detected mutations in 40% families by analyzing all of the 12 genes. Overall, the frequencies of mutations in the 12 genes varied significantly in different ethnic populations but were more likely to be less than three fourth of families with congenital cataracts. The overrepresentation of mutations in these 12 genes is more likely due to more frequent studies on these genes. Although several mutations in the 12 genes have been identified so far, we still identified five novel mutations in the Chinese families with congenital cataracts. Our results expand the mutation spectrum of these genes in Chinese population. The families without identified mutations will be good candidates for future study in screening for additional causative genes.